The present invention relates generally to measuring the characteristics of a telephone cable and more specifically to determining the number and location of load coils in a telephone line using the impedance characteristics of the telephone line.
Traditional voice telephony uses load coils on very long twisted-pair lines to improve the frequency response of the lines. The most common loading pattern uses 88 mH coils with a nominal spacing between the coils of 6,000 feet and 3,000 feet at each end. In situations where one or more load coils are missing or shorted, due to improper removal, lightening strikes, and the like, it becomes important for a technician to easily determine the missing or malfunctioning load coil(s) in the line to minimize service time.
Traditional methods of locating missing load coils include long pulse TDR and network synthesis. The long pulse TDR method uses a time domain reflectometer that launches long pulses into a twisted-pair line and generates waveform data that is displayed as a magnitude versus distance waveform trace on a display device, such as a cathode-ray-tube or liquid crystal display. Long pulses are used because the bandwidth of the twisted-pair line is in the range of 4.5 kHz. Because the pulsewidths of the launched pulses are greater than the spacing between the load coils, it is difficult for a user to determine the existence and location of load coils in the line. Load coils appear on the waveform trace as subtle changes in the slope of the trace. Accurately identifying the subtle changes and where they start, especially in the presence of noise, is very difficult, even for an experienced technician.
The network synthesis method measures the magnitude versus frequency response of the line to try and extract the pole and zero frequencies of the input immittance of the line. Using classical network synthesis equations on the pole and zero frequencies, and assuming the line resistance is zero, a first Cauer L-C ladder network is synthesized to approximate the loaded line. Letting the L values represent the load coil inductance and C to represent the length of the line between each load coil, the network synthesis method provides the needed information when the pole and zero frequencies are measured accurately. U.S. Pat. Nos. 4,087,656 and 4,087,657 respectively describes acquiring immittance magnitude values and immittance magnitude and phase values as the frequency of an input signal is swept and looking for peaks and valleys in the data to estimate the pole and zero frequencies. A ladder network is synthesized corresponding to the estimated poles and zeros. Once the ladder network is synthesized, any missing elements can be determined and related back to the causal missing load coil. The main disadvantages of this method include computational complexity, the need for specialized phase measurement circuitry when the phase is measured, and the difficulty of locating pole and zero frequencies when the cable has resistance.
A further method described in the Background of the Invention of the 656 patent employs a piece of test equipment that includes a cathode ray tube for displaying a pattern representing, as a function of frequency, the impedance characteristics of the line under test. At the same time, the tube displays a second pattern which represents, as a function of frequency, the impedance characteristics of a lattice network. This network is made up by the test equipment operator from a kit to cause the second pattern to approximate that of the line under test. When a good match between the two patterns is achieved, then the configuration and values of the network elements are indicative of the composition of the line under test. The patent states that although the equipment had been used successfully, it was found that producing a reasonable match of patterns was time consuming and somewhat of an art.
What is needed is an apparatus and method for determining the number of load coils that should be present in a properly loaded telephone line and positionally detects which, if any, load coil(s) are missing. Such an apparatus and method should be portable, easy to use, and sufficiently fast to allow field technicians to quickly locate missing or malfunctioning load coil(s) and effect repairs.